We demonstrate a free-space amplitude modulator for mid-IR radiation ( λ around 10 µm) that can operate up at least 400 MHz (-3dB cut-off at ∼100/150 MHz) and at room-temperature. The device is based on a semiconductor hetero-structure enclosed in a judiciously designed optical resonator based on a metallic meta-surface. At zero bias, it operates in the strong light-matter coupling regime up to 300K. By applying an appropriate bias, the device transitions to the weak coupling regime: the important change in reflectivity due to the disappearance of the polaritonic states is exploited to modulate the intensity of a mid-IR laser source up to at least 400 MHz .
We show that intersubband (ISB) polaritons are robust to inhomogeneous broadening. In a series of multiple quantum well samples with mid-infrared ISB absorption transitions with broadenings varying by a factor of 5 (from 4 meV to 20 meV), we observed polariton linewidths lying in the 4 - 7 meV range only. We have verified the dominantly inhomogeneous origin of the broadening of the ISB transition, and that the linewidth reduction of the modes persists up to room-temperature. The immunity to inhomogeneous broadening is a direct consequence of the coupling of the large number of ISB oscillators to a single photonic mode.
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